184 Sex-limited hiheritance [ch. 



non-pigmented kinds, with many puzzling intergradations 

 between them. 



Leaving aside all that is obscure in this experiment for 

 future consideration one fact stands out clearly as an almost 

 inevitable conclusion from the data, namely that in addition 

 to the sexual differentiation into male and female, and the 

 racial differentiation into pigmented and non-pigmented, the 

 operation and interference of some unknown third kind of 

 differentiation has to be reckoned with. In no other way 

 can the results from the reciprocal crosses between the pure 

 types, and those from the non-pigmented hens bred to the 

 F^ male, be brought into harmony. It is the appearance of 

 the one deeply-pigmented female in eight birds which gives 

 the clue. 



Since half the gametes of the F^ male must be bearing 

 pigmentation, and since from results of the mating Brown 

 Leghorn x Silky male we know that a pigment-bearing 

 gamete from the male may dominate in the female, it would 

 be expected that half the female offspring would be deeply 

 pigmented ; for certainly half of them contain the necessary 

 element. But as only a quarter of the female offspring are 

 of the deeply-pigmented class there must be some other 

 element present which obliterates the pigmentation, or holds 

 it in check in the missing quarter. In several cases we are 

 well aware of the existence of such inhibiting factors, for 

 example that which causes the flower of a Chinese Primula 

 to be white though the factors for colour are present in it 

 (see Chap, v, p. 105). 



This factor may be spoken of as Z>. In the Silky it is 

 evidently not present and therefore it must come from the 

 Leghorn. Since Silky % x Brown Leghorn ^ gives no 

 deeply-pigmented offspring we must consider the Brown 

 Leghorn cock to be homozygous in D . But as the Brown 

 Leghorn % x Silky ^ gives the female offspring deeply 

 pigmented, the Brown Leghorn hen must be heterozygous 

 in D, and there must be some system or mechanism by 

 which this factor D descends to her male offspring and not 

 to her female offspring^. 



* A priori it might be thought possible that the dominance of the 

 pigmentation-element in the F-^ ? was due to some special differentiation 

 of half the gametes of the male. On the hypothesis that each sex is 

 heterozygous for sex such a system might without improbability be con- 



